This invention relates to clock signals in optical systems, particularly to transmitting a clock signal in soliton optical transmission system.
In very long distance optical fiber links, it is known to use a soliton type of optical signal to minimize the effects of chromatic dispersion on the signal due to the dispersive properties of the fiber. A soliton type of signal makes use of the way in which the refractive index of the fiber varies with signal intensity in order to offset the dispersive effects, thereby preserving the spectral form of the signal as it propagates along the fiber.
In a transmitter of the system, a group of data channels are time division multiplexed into a single channel, typically having a bit rate of 100 Gb/s, and the information in the single channel is transmitted over the fiber by the soliton signal. In the optical link, in-line optical amplifiers, such as erbium doped fiber amplifiers (EDFA), amplify the soliton signal to compensate for line losses of the link. Regenerators may also be used, especially in very long links, to recreate the original soliton signal, thereby removing effects from propagation and amplification, such as timing jitter, noise, and minimal spectral dispersion. At the end of the link, a receiver demultiplexes the data channels from soliton signal.
Both the regenerator and the receiver require a clock signal at the full line rate, 100 Gb/s, in order to perform their functions. Further, the regenerator would require a 100 Gb/s electrical clock signal in order to regenerate the soliton signal without time division demultiplexing it into the separate data channels. However, generating a 100 GHz electrical clock signal from a 100 Gb/s soliton signal presents a problem because opto-electronic convertors (i.e. PIN diodes) that can operate at such a frequency are not available now, nor are they likely to become available in the near future. Furthermore, all-optical solutions for generating a 100 GHz clock signal are unattractive because of their complexity, size, and lack of stability.
An object of the present invention is to provide method and apparatus for communicating a clock signal in a soliton optical transmission system.
The invention provides a stream of soliton optical pulses having a spectral line in their frequency spectrum at the line rate of the pulses divided by an integer (N). To this end, the stream of soliton pulses is modulated with the clock signal, which is an Nth sub-harmonic of the line rate of the soliton pulses. Accordingly, the average energy of the pulses in every Nth time slot is distinct from the rest of the pulses in the other time slots. This distinction in the average energy can be detected, thereby allowing recovery of the clock signal. For example, in a 100 Gb/s soliton system, the pulse in every fourth time slot would be modulated thereby corresponding to a 25 GHz clock signal.
An advantage of communicating the clock signal by modulating the stream of soliton pulses is that it does not use additional fiber bandwidth, hence it does not adversely effect the throughput of the soliton system. That is, it does not reduce the rate at which soliton pulses can be transmitted across the fiber optic link.
Conveniently, the frequency of the clock signal is within the frequency range of currently available PIN diodes. An advantage of using a sub-harmonic clock signal of such a frequency is that an electrical clock signal can be easily obtained. Furthermore, the electrical clock signal can be up-converted to the full line rate for use in a regenerator. Moreover, such a solution is more attractive than all-optical solutions because it is less costly and less complex.
According to the invention the average energy in the Nth time slot is made distinct in a way that does not effect the propagation properties of the soliton pulses, thereby maintaining all the benefits that they provide. That is, modulated pulses in the Nth time slot remain soliton pulses and therefore they propagate along the fiber link as such. Accordingly, the pulses in the Nth time slot are modulated to change either their width-to-amplitude aspect ratio, their position within the time slot or their polarization. Alternatively, binary data symbols carried by the stream of soliton pulses could be encoded such that the data symbol carried by the pulse in the N/2th time slot has a probability greater than 0.5 of being the converse of the data symbol in the previous N/2th time slot. This encoding would also create a spectral line in the frequency spectrum of the pulses at the line rate divided by N.
According to an aspect of the present invention there is provided a transmitter for transmitting optical soliton pulses and providing a clock signal via the optical soliton pulses in a soliton optical transmission system comprising an optical soliton pulse source for generating optical soliton pulses at a first rate, the first rate defining time slots of equal duration; and a modulator for modulating each optical soliton pulse in every Nth time slot in a manner such that each said optical soliton pulse is distinguishable from optical soliton pulses in other time slots, whereby the clock signal has a frequency equal to the first rate divided by N, where N is an integer greater than one.
According to another aspect of the present invention there is provided a transmitter for transmitting optical soliton pulses over an optical fiber in a soliton optical transmission system comprising an optical soliton pulse source for generating optical soliton pulses at a first rate, the first rate defining time slots of equal duration; a data source for providing data symbols at the first rate; a plurality of modulators for modulating the optical soliton pulses in dependence upon the data symbols provided by the data source; and a first modulator of the plurality of modulators for modulating each optical soliton pulse in every Nth time slot in a manner such that said each optical soliton pulse is distinguishable from optical soliton pulses in other time slots.
According to another aspect of the present invention there is provided a receiver for receiving optical soliton pulses arriving at a first rate from an optical fiber in a soliton optical transmission system comprising means for recovering a clock signal from the optical soliton pulses, the clock signal having a frequency equal to the first rate divided by an integer N, wherein the integer N is greater than one; and a demultiplexer for demultiplexing the optical soliton pulses into a number of streams of optical soliton pulses responsive to the recovered clock signal, the number of streams being an integer multiple of the integer N.
According to another aspect of the present invention there is provided a method of encoding a clock signal in a soliton optical transmission system, the method comprising the steps of generating a series of optical soliton pulses at a first rate, the rate defining time slots of equal duration; and modulating each optical soliton pulse in every Nth time slot in a manner such that said each optical soliton pulse is distinguishable from optical soliton pulses in other time slots, where N is an integer greater than one.
According to yet another aspect of the present invention there is provided a method of recovering a clock signal in a soliton optical transmission system from a series of optical soliton pulses transmitted at a line rate, the clock signal having a frequency equal to the line rate divided by an integer N, wherein the integer N is greater than one, the method comprising the steps of receiving the optical soliton pulses; converting the optical soliton pulses to an electrical signal; filtering the frequency of the clock signal from the electrical signal to provide a filtered clock frequency signal; and amplifying the filtered clock signal frequency signal to provide a recovered clock signal.
According to still another aspect of the present invention there is provided a signal in an optical soliton transmission system comprising a series of optical soliton pulses transmitted at a line rate, the line rate defining time slots of equal duration, and each optical soliton pulse in every Nth time slot being distinguishable from optical soliton pulses in other time slots to encode a clock signal having a frequency of the line rate divided by N, where N is an integer greater than one.